1. Field of the Invention
This invention relates to a focus detecting device, and in particular to a focus detecting device suitable for use in a photographic camera, a video camera or the like in which a plurality of pairs of object images are formed by a plurality of pairs of secondary imaging lenses disposed rearwardly of an objective lens and discrimination of the in-focus state of the objective lens is effected from the amount of deviation between each pair of object images. The invention also relates to a method of manufacturing such a device.
2. Related Background Art
In single-lens reflex cameras or the like, use has heretofore often been made of a so-called image deviation type focus detecting device in which, on the basis of light fluxes from two different portions of the pupil of an objective lens, two object images are formed on a photoelectric conversion element array by a pair of secondary imaging lenses. The focus state of the objective lens is then detected from the relative positional relation between these object images.
However, the direction of arrangement of the pair of secondary imaging lenses is limited to one direction. Therefore, for example, in the case of an object having a pattern in a direction orthogonal to the direction of arrangement of the photoelectric conversion element arrays disposed correspondingly to the secondary imaging lenses, focus detection is possible. However, in the case of an object having only a pattern in the direction of the photoelectric conversion element arrays, any change in the object image cannot be detected and thus, focus detection becomes impossible.
Also, where a focus detecting device of the above-described construction is used as the focus detecting device of an interchangeable lens type camera such as a single-lens reflex camera, the distance between the vertices of a pair of secondary imaging lenses is also set in accordance with a dark lens of great F-number. Therefore, even when a bright lens of small F-number which inherently requires highly accurate focus detection is mounted to the camera, the limit of focus detection accuracy becomes the same as that when a dark lens is mounted to the camera, and highly accurate focus detection becomes difficult.
In order to solve the above-noted problems, a method of installing a plurality of pairs of secondary imaging lenses has been proposed in U.S. application Ser. No. 313,343. Also, devices in which the directions of arrangement of two pairs of secondary imaging lenses are orthogonal to each other, as shown in FIG. 7 of the accompanying drawings, have been proposed in U.S. application Ser. No. 102,622 and U.S. Pat. No. 4,774,539. There is also conceivable a device in which, as shown in FIG. 8 of the accompanying drawings, the directions of arrangement of two pairs of secondary imaging lenses are the same, but the distances between the vertices of these two pairs of secondary imaging lenses differ from each other. In FIG. 7, lenses 11 and 12 and lenses 13 and 14 form respective pairs, and in FIG. 8, lenses 111 and 121 and lenses 131 and 141 form respective pairs.
However, when manufacturing a member constructed of said two pairs of secondary imaging lenses, if the secondary imaging lenses are molded as a unit or cemented together, the degree of perpendicularity and the degree of parallelism of two straight lines passing through the vertices of the two pairs of lenses will fail to be strictly 90 degrees and 0 degree, respectively, and a manufacturing error, though slight, will occur.
In contrast, as regards the arrangement of photoelectric conversion elements, the manufacturing error is very small and its influence upon the function of the device is usually negligible.
FIG. 9 of the accompanying drawings is an illustration expressing an exaggerated lens state in which the degree of perpendicularity of the two pairs of secondary imaging lenses is spoiled though slightly. In FIG. 9, lenses 11 and 12 and lenses 132 and 142 form respective pairs.
Here, when the two pairs of secondary imaging lenses whose degree of perpendicularity is spoiled is in the state as shown in FIG. 9, if an attempt is made to photograph an object comprising, for example, edges of 45.degree. with the direction of photoelectric conversion element arrays being adjusted to the vertices of the lenses 11 and 12, the positional relation between the object image and the photoelectric conversion element arrays will be such as shown in FIG. 10 of the accompanying drawings. FIG. 10A shows the positions of the object images when an error in the degree of perpendicularity at which the directions of arrangement of the two pairs of secondary imaging lenses are orthogonal to each other occurs as shown in FIG. 9. FIG. 10B shows the positions of the object images when in the state shown in FIG. 7 in which there is no error in the degree of perpendicularity at which the directions of arrangement of the two pairs of secondary imaging lenses are orthogonal to each other. In FIG. 10, the reference numerals 16 and 17 and the reference numerals 18 and 19 designate pairs of photoelectric conversion element arrays provided to receive the object images formed by the lenses 11 and 12 and lenses 13 and 14 of FIG. 7. It will be seen that the distance between the object images on the photoelectric conversion element arrays 18 and 19 differs between al, a2 of FIG. 10A and bl, b2 of FIG. 10B and therefore accurate focus detection cannot be accomplished for such a pattern which is oblique to the photoelectric conversion element arrays as in this example.
This also holds true of the two pairs of secondary imaging lenses as shown in FIG. 8, and focus detection will become inaccurate unless the degree of parallelism of the straight line passing through the vertices of the lenses 131 and 141 to the straight line passing through the vertices of the lenses 111 and 121 is substantially zero degree without being spoiled by a manufacturing error.